1. Field of the Invention
The present invention relates to a high resolution lens, and more particularly, to a high resolution parabolic lens which is capable of reducing the size of an optical spot to below a diffraction limit without using another condensing lens.
2. Description of the Background Art
Resolution of a microscope, a line width of a semiconductor exposing equipment, and a storage density in an information storage device have a direct relation with the size of a light spot.
That is, as the light spot becomes small, its resolution is increased, the line width of a semiconductor circuit can be reduced and the information storage density of the information recording apparatus can be increased.
The size of the light spot depends on the wave length of a light, a light convergency angle of a lens, a diameter of an incident beam, a refractivity of a medium on which a spot is formed, which can be expressed by the following formula:
d=0.6xcex/n*sinxcex8
where xe2x80x98dxe2x80x99 indicates the diameter of a spot, xcex indicates a wavelength of a light, xe2x80x98nxe2x80x99 indicates refractivity, and xcex8 indicates a convergency angle.
Accordingly, in order to reduce the light spot, the wave length is to be reduced, the refractivity is to be great and the convergency angle is to be great. However, the size of the light spot is limited to one-half of the wave length of the used light due to the-diffraction limit.
Accordingly, in order to reduce the light spot, the wave length is to be reduced, the refractivity is to be great and the convertency angle is to be great. However, the size of the light spot is limited to a half the wave length of the used light due to the diffraction limit.
Therefore, in an effort to overcome the diffraction limit and obtain a smaller light spot, there have been proposed several methods.
Blue laser light is used to reduce the wave length of the light, so that the wave length of about 600 nm can be reduced to about 400 nm.
In addition, a method has been proposed in which a lens having a high refractivity is used to reduce the spot. A solid immersion lens (referred to as SIL, hereinafter) is its typical example.
FIG. 1 illustrates an optical system in use for an information recording apparatus.
As shown in the drawing, the optical system includes a hemispherical SIL 12 and a first condensing lens 10.
The SIL 12 is adjacent to a recording medium 14 and has a hemispherical shape of which the upper surface is spherical and the lower surface is plane. The center of the plane portion of the SIL corresponds to the focus of the first condensing lens.
The light made incident on the first condensing lens is deflected and proceeds into the SIL. Since the focus of the first condensing lens exists at the center of the plane portion of the SIL, the incident light is collected at the center of the plane portion of the SIL.
The light convergency angle (xcex8) is determined by the numerical aperture of the first condensing lens, and refractivity (n) is determined according to the material of the SIL. The maximum refractivity of the material used for the SIL is about 2.2, and the sine value (sinxcex8) of the convergency angle by the numerical aperture of the first condensing lens is possibly by 0.7. Therefore, in case of using a light of which wave length (xcex) is 632 nm, the minimum spot size xe2x80x98dxe2x80x99 realizable with the SIL is about 246 nm.
In order to record a data (bit) on a disk by using the SIL, as shown in FIG. 2, the SIL 20 approaches the surface of a recording medium 22, leaving a space (26) of about 10xcx9c70 nm.
As the SIL 20 is approaching, a light near-field phenomenon occurs that the light energy first collected at the lower surface of the SIL is partially transmitted to the recording medium.
Thanks to the near-field phenomenon, a data can be recorded or reproduced on or from the surface of the recording medium. For example, the energy transmitted from the SIL heats a portion of the surface of the recording medium, causing a partial phase change, and a bit 24 is formed on the surface of the recording medium owing to the phase change, that is, data is recorded.
In case where a recorded data is read, a characteristic that the reflectivity is changed at the portion where the phase is changed is utilized.
That is, a light having a low intensity compared to the case of recording is made incident on the surface of the recording medium through the SIL, and the intensity of the light coming out through the SIL after being reflected on the surface of the recording medium is measured. Then, since the reflectivity is different according to the existence and non-existence of the bit, the data can be read.
As stated above, the optical system using the SIL overcomes the diffraction limit of the light as well as reducing the light spot, but also has the following problems.
Generally, as shown in FIG. 3, the optical lens 30 has a problem of aberration that light is not collected at one point. The aberration becomes great as the magnification of the lens is high. In case of the optical system using the SIL, since it needs the first condensing lens, the aberration of the first condensing lens considerably degrades the first collecting capacity of the optical system.
In addition, in case of the hemispherical SIL using the near-field, since it needs to approach the recording medium or an object for measurement, there is a possibility of collision. The SIL has a plane bottom surface having a diameter of about 1 mm and should maintain the space of 50 nm with respect to the recording medium. Thus, since the plane face (having a diameter of 20000 times of the space) is relatively wide compared to the space, if the SIL is a bit inclined to the recording medium, it would collide the latter.
Moreover, the data recording and reproducing apparatus using the SIL needs the first condensing lens, causing- the apparatus to have a large volume and to be complicated. Also, it is difficult to assemble the whole data storing unit and the first condensing lens.
Therefore, an object of the present invention is to provide a high resolution near-field optical system directed to reducing a possibility of collision between a near-field optical system and a recording medium or an object for measurement.
Another object of the present invention is to provide a high resolution near-field optical system without having a first condensing lens.
Still another object of the present invention is to provide a high resolution lens of which light-collecting performance is remarkably enhanced by reducing its aberration.
Yet another object of the present invention is to fabricate various apparatuses simply by using the high resolution lens.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a high resolution lens for collecting light made incident from an external source, of which one surface is plane and other surface is curved and coated with a reflecting material except for its central top point (apex).
The curved face of the high resolution lens may be spherical or is preferably a parabolic face to reduce aberration.
When a parallel light is made incident on the plane face of the high resolution lens, it is reflected in the lens and totally reflected, and then is transmitted through the uncoated apex to an external object.
Also, the present invention introduces various techniques adopting the high resolution lens, including the information recording apparatus.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.